The present invention relates in general to a highly reflective painting product. The invention has been developed in particular, but not exclusively, in connection with a painting product suitable as a coating for integrating spheres of spectrophotometers and the like, the spheres being intended for collecting and measuring luminous radiation reflected diffusely in all directions by a generic sample. Naturally, the use of the above-mentioned painting product may extend to other uses, for example, but not exclusively, for coating the surfaces of standard samples of known reflectance which are used, for example, for calibrating spectrophotometers, or as a coating for spheres which constitute sources of amplified and homogeneous light. Other possible uses of the product which, again, are given by way of non-limiting example, relate to the lighting design field and comprise the coating of covers, panels and diffusive reflective surfaces for lighting in general.
In general, integrating spheres collect and spatially integrate a radiant flux. The flux may be measured directly or after it has interacted with a sample of material. The sphere, as part of a radiometer or photometer, can directly collect and amplify the flux originating from lamps or lasers, or the flux density produced by a hemispherical illumination. Perhaps the most important application of integrating spheres is for the measurement of the total reflectance or of the transmittance of diffusive or dispersive materials. An alternative application uses the holes in an integrating sphere, which is illuminated internally, as a source of uniform radiance. These sources can be used for calibrating electronic image devices and systems or simply as uniform back-lighters.
It has been known for some time to use, in spectrophotometers, integrating spheres each having a spherical internal cavity coated by a highly diffusely reflective layer or coating film by virtue of which the luminous flux of a source can be amplified, rendering it homogeneous and isotropic. An integrating sphere of this type generally has an opening in front of which a sample can be placed, the sample reflecting into the sphere a light beam which in turn is reflected and diffused by the wall of the internal cavity of the sphere and is measured by sensors of known type.
As mentioned, for example, in the documents U.S. Pat. Nos. 3,764,364, 4,035,085 and EP-0511806, it has been known for some time to coat or paint the internal cavities of integrating spheres with products based on barium sulphate, magnesium oxide and/or magnesium carbonate, whereas, more recently, long-chain fluorinated aliphatic addition-polymer films have been proposed. The known products based on barium sulphate present a series of problems, amongst which sensitivity to moisture and poor mechanical strength, which cause the coating layers of the integrating spheres treated with these products to deteriorate easily, are known. Moreover, the known products based on barium sulphate have poor resistance to ultraviolet radiation and tend to age rapidly, so that their reflectance characteristics are reduced.
The above-mentioned addition-polymer films in turn have been found difficult to apply to the wall of the internal cavity of an integrating sphere. To solve this latter problem, the document EP-0511806 proposes the solution of forming the entire body of an integrating sphere of this polymer material so as to eliminate all of the problems of adhesion of the polymer film at their root. However, the solution proposed appears rather unsatisfactory since an integrating sphere thus formed is not completely opaque, particularly when it has small dimensions, and some of the luminous flux shines through to the outside of the sphere, thus reducing its overall efficiency.
The Applicant has therefore performed research directed towards identifying a conventional coating product, for example, based on barium sulphate or the like, to be applied to the internal cavity of an integrating sphere of non-transparent material, for example, aluminium. The above-mentioned conventional coating products comprise two basic elements: a reflective material such as barium sulphate, and a binder or glue which serves to give the reflective material cohesion. These two ingredients are generally supplied in suspension in a liquid vehicle, for example, a mixture of water and alcohol, suitable for enabling the coating to be applied, for example, by spraying.
The product is applied to the desired surface, after which the liquid vehicle evaporates, leaving a compact layer of reflective material, rendered coherent by the binder, adhering to the surface. As with barium sulphate, the reflective material is used in powder form with a very fine particle size (a mean diameter of approximately 1-2 microns). In particular, barium sulphate has optimal inertness with respect to the chemical attacks to which it may be subjected in a normal working environment.
Examples of known coating products are Eastman Kodak Company's product known by the trade name KODAK White Reflectance Coating and Labsphere Inc.'s product known by the trade name WRC-680 White Reflectance Coating. Both of the above-mentioned known products contain barium sulphate bound by a glue based on polyvinyl polymer which, however, tends to lose its transparency characteristics and to age over time, turning yellow and reducing reflectance in the blue region of the light spectrum, with exposure to light. In order to reduce the significance of the ageing effects, it would be necessary to reduce the percentage of glue in the coating product, to the detriment of the mechanical cohesion of the coating product. Basically, if too much glue is used, the coating tends to yellow after a certain period of time, whereas if too little is used, the coating tends to become “floury” and to become detached from the surface to which it is applied.
In order to be able to apply the reflective material in powder form and the binder to the surface to be coated, it is necessary to disperse these ingredients in a liquid vehicle which must have good fluidity in order to be easily sprayable. The fluid which acts as the vehicle must also evaporate completely, quickly enough not to lengthen the application times but not too quickly in order not to create problems during application, which is usually performed by means of a compressed-air spray gun which may become blocked.